A substrate having a semiconductor layer on an insulating layer is known as an SOI (Silicon On Insulator or Semiconductor On Insulator) substrate. A method of manufacturing an SOI substrate is disclosed in Japanese Patent Laid-Open No. 5-211128. In the method disclosed in this reference, to separate a semiconductor substrate into a lower portion and a thin upper portion, ions are implanted into the semiconductor substrate. Next, the semiconductor substrate is bonded to a stiffener. The assembly of the semiconductor substrate and stiffener is annealed to rearrange the crystal and coagulate bubbles in the ion-implanted layer, thereby separating the lower portion from the thin upper portion. With this method, the thin upper portion of the semiconductor substrate is transferred to the stiffener.
The reference discloses that a semiconductor substrate having an encapsulating layer made of, e.g., silicon oxide is employed as a semiconductor substrate in which ions should be implanted, and the encapsulating layer may be either removed or left after ion implantation. However, the reference neither discloses nor suggests any technique for thinning down the silicon oxide encapsulating layer after ion implantation.
The present inventors are focusing attention on an SOI substrate manufacturing method. In this method, an insulating layer is formed on the surface of a semiconductor substrate. After that, ions are implanted into the semiconductor substrate through the insulating layer to form an ion-implanted layer. The semiconductor substrate is bonded to another substrate to prepare a bonded substrate stack. Then, the bonded substrate stack is split at the ion-implanted layer.
In this method, however, the thicknesses of the SOI layer and the underlying buried oxide film (BOX layer) in the resultant SOI substrate are controlled by the depth of ion implantation in the semiconductor substrate. When a thin BOX layer should be obtained, the SOI layer becomes thick. Conversely, when a thin SOI layer should be obtained, the BOX layer becomes thick. To make both the SOI layer and the BOX layer thin, the ion implantation energy must be low. In this method, however, it is difficult to accurately control formation of the ion-implanted layer or stably form the ion-implanted layer.